1. Field of Invention
This invention relates in general to drilling bit design optimization and selection for use by the oil and gas industry, and in particular to systems, program product, and related methods for selecting and designing a drilling bit tailored for a target section of the earth.
2. Description of the Related Art
Drilling is essential in civil, mining, and petroleum industries. It is also a pre-requisite in exploration and exploitation of oil, gas, and other energy resources. With the depletion of shallow energy resources, however, the cost of drilling is getting increasingly higher; especially, when harder rock formations are encountered and where higher rate of penetration is desired. Therefore, in order to minimize the cost of drilling, it is important that the conditions for optimum performance of drilling are identified.
The optimum performance of drilling depends on large number of factors; most importantly rock type, bit type, rock-bit interactions, hydraulics, stability, and the type of drilling system employed. Over the years, a significant advancement has taken place in understanding of these subjects.
Knowledge of rock and rock-bit interations, however, remains a weak link in drilling. The poor understanding of rock and its interaction with drilling bits primarily stems from the fact that it is an interdisciplinary subject. It requires at least some expertise in the field of geology of rock formations, chemistry of minerals and its bond structures, physics of force or energy application, rock mechanics aspects of the deformation process, and fracture mechanics aspects of the failure process. Moreover, it is difficult, if not sometimes impossible, to mimic the actual drilling process in rocks taking place in the original environments of boreholes. Thus, recognized by the inventors is the need for additional work on rock and its interactions with the drill bit to further enhance the understanding of optimum performance of the bit and drilling system.
In any drilling or cutting process, energy is applied through the cutting tool to generate stresses in the working surface. The stresses may be classified as compressive, shear or tensile or some combination of these. If the stresses are sufficiently high, some type of cutting action occurs, such as crushing, scraping, chipping or indenting. In the drilling industry, the ease at which a given rock can be crushed, scraped, chipped or indented is known as its drillability. Drillability depends on elastic and strength properties of the material. In particular, the unconfined compressive strength (UCS) of rock is recognized in the industry as an important if not sole factor to be used in determining its drillability. Realistically, however, drillability involves a large number of factors, including physical, mechanical, and micro-structural properties. The micro-structure of rock becomes particularly important when the size of the chips generated are near the grain size level. The macro-structure (e.g. bedding planes, interbeddedness, rattiness characterized using log data) is often times absent in relatively small diameter drilling holes, but if present it can significantly influence the drilling process. These and other factors affecting drillability, however, are often overlooked such that drillability of rock is often inaccurately expressed solely in terms of its UCS. There is, therefore, a need in the art for an efficient and systematic way to express the drillability of rocks comprehensively. This can be particularly important with respect to carbonate rocks.
The microstructure of rock is defined by its mineral components, grain shapes, sizes and their interlocking or packing (a.k.a. texture). Minerals are naturally occurring inorganic compounds and are present in the grains, grain-boundaries and as cementing material between two or more grains. Contrary to the fixed proportions of atoms, molecules or ions, as dealt with in chemistry, rocks are mainly solid solutions of silicates, carbonates, oxides, etc. Some of the inorganic elements get replaced in course of rock formations, and thus, the elements are typically represented in parenthesis to represent rock, as nearly as possible. The exact portion, however, is difficult to present. The individual minerals get crystallized and get crowded until they make a rigid mass of shapeless lumps called grains. Most of the minerals constituting rocks are silicates, e.g., quartz, feldspar, mica, clay, calcite, dolomite, olivine, garnet, pyroxenes, and amphiboles. The basic building block of silicate is Silicon-oxygen tetrahedron being linked in a single- or double-chain, sheets, a three-dimensional network, or not linked at all. Their bond structure, in combination with the characteristic cleavage property, makes a rock strong or weak. Minerals in sedimentary rocks are typically carbonates in addition to quartz and clay. Sulphate minerals are typical in gypsum and anhydrite.
About 99 percent of the sedimentary rocks consist of quartz, clay and carbonates resulting from sedimentary processes, such as weathering, transportation, sorting, and deposition, compaction and its typical diagenesis, up to the present age. The dominance of these individual minerals depends on the location where they were formed. For example, sandstone mainly consists of broken pieces, well sorted, un-weathered quartz. The fine sand and clay are transported more in suspension as they travel down the stream, thus forming shale. The dissolved portion of the lime and carbonate travels much further with the water. The calcite is deposited because plants and animals extract it from sea water and use it to build their skeleton. The other common rocks associated with shale sandstone and carbonates include coal, salt, gypsum, phosphate, chert and conglomerate. A large body of knowledge exists in the literature dealing with clastic rocks such as sandstones. As a result there are several models which can be used to predict the behavior or rocks from one basin to other, albeit with limited success. In contrast, relatively few works have been accomplished dealing with carbonate rocks.
Carbonate rocks, in general, are significantly different than other sedimentary rocks of sandstone or shale, due to their typical diagenesis processes including compaction, cementation, precipitation, dissolution, re-crystallization, dolomitization, or replacement of some constitutive minerals or fluids by other elements in the space available including in and around grains. Due to these typical diagenesis processes, porosity may be either reduced (dolomitization causes shrinkage by ˜12.5%), or enhanced (moldic porosity, fracturing, vug or cavity formation), and/or a discontinuity may be formed as in stylolytes, like the horizontal layers seen in Carthage marble, or extended as in caverns or vugs. For example, in the Jurassic Arab limestone of Ghawar field in Saudi Arabia, replacement has caused a reduction in primary porosity. In the Jurassic Smackover Formation of Alabama and the Leduc reef carbonates in Alberta, porosity and permeability were preserved due to the existence of a rigid framework formed during early dolomitization. In general, however, dolomitization enhances porosity because dolomites are denser, and consequently, take up less volume than the original calcite. Accordingly, recognized by the Applicants is that lessons learned in one carbonate rock does not apply to others, and that such lessons are case specific, in contrast to lessons learned with respect to the majority of sandstone and shale rocks. There is, therefore, a need for an efficient and systematic way to ascertain drillability of each significant rock formation for a specific target hole section.
Conventionally, carbonate rocks have been characterized by providing qualitative rather than quantitative information. That is, typical carbonate rock characterization consists of communicating as much information as possible with respect to the depositional environments of carbonates and its evolutions, together with its constitutive mud, cement and grain network. These are gathered from testing the rock with dilute hydrochloric acids, and observing the rock under binocular microscopes, or hand lenses. Traditionally, Folk (1959) and Dunham (1962) are the only two classification systems for characterizing carbonates. Some of the recent work by Akbar et. al. (2001) and Embry and Klovan (1971), however, enhance and clarify the classification systems with more details of grain sizes and pore systems in albeit a qualitative way. Further, the work of Choquette and Pray (1970) and Lucia (1983) adds to the aspects of porosity and grain sizes, but with limited success due to complex nature of carbonates. Further, some of the recent works on above classifications have added grain sizes, pore types and porosity which altogether clarify and characterize carbonates in a much better way; albeit still in a descriptive way. Accordingly, unlike sandstones which are characterized by grain sizes only, there is no general procedure to characterize carbonate rocks (in general), as the carbonate rocks of one place may be completely different than that found at other places, and as the lessons learned on one carbonate rock would not be able to be readily used for other carbonates. There is, therefore, a need for an efficient and systematic way to quantitatively characterize and evaluate rocks (including carbonate rocks) on a case-by-case basis in order to objectively evaluate drillability.
In the oil and gas industry, drilling bits such as, for example, polycrystalline diamond compact (PDC) drilling bits, are generally selected based on design features. These design features can include, for example, blade count, cutter count, cutter size, gauge design (length, geometry), nozzle count, face volume, and junk slot area. Performance characteristics and not design features, however, directly determine the success of a design in a given application. Examples of performance characteristics can include the level of dynamic stability (e.g. torsional stability, lateral stability), axial and lateral aggressiveness, mechanical efficiency, cleaning efficiency, cooling efficiency, erosion resistance, impact resistance, and abrasion resistance. As a result of the focus on design features rather than performance characteristics, bit selection has involved a trial-and-error approach. There is, therefore, a need in the art for an efficient and systematic way to capture and compare performance characteristics for drilling bits to performance requirements associated with a target hole section of rock to facilitate bit design optimization and selection.